(1) Field of the Invention
This invention relates to direction finding, and more particularly to direction finding using radio techniques.
(2) Description of the Art
Direction finding by radio is known. In a direction finding system having a number of separate antennas, radio wavefronts reach the antennas with different delays. Assuming the wavefronts have narrow bandwidth, these delays give rise solely to relative phase shifts. For determining the directions or bearings of M−1 radio frequency (RF) emitters (M is a positive integer), a typical direction finding system employs M antennas, a respective receiver for each antenna, and processing circuitry for each receiver. The processing circuitry implements a discrete Fourier transform to divide its respective signal into frequency bins or channels. Signals are then combined in pairs, and for each channel a respective covariance matrix is constructed from which emitter bearings are estimated. This approach suffers from the problem of requiring as many receivers as there are antennas, receivers being expensive and bulky. It is unsuitable for example for man-portable equipment or for mounting in small aircraft.
To get round the multiple receiver problem, it has been proposed to use a single port receiver, spatial spectrum estimation technology, and a weight perturbation algorithm to obtain the covariance matrix. See Zhao Yimin, “A Single Port Receiver Spatial Spectrum Estimate DF System”, 0-7803-3216-4/96 IEEE 1996. This is however a relatively complex approach to the problem.
It has also been proposed to combine RF antenna signals in beamformers and use a single receiver connected to successive beamformers via a multipole switch, one pole per beamformer. See C M S See, “High Resolution DF with a Single Channel Receiver”, 0-7803-7011-2/01 IEEE 2001. A single spatial covariance matrix is formed, and the bearings or directions of up to M−1 emitters can be estimated, where M is the number of antennas. This requires M2 beamformers, another source of expense and bulk.
Another solution is adopted in the Rohde & Shwartz DDF 195 instrument, which combines pairs of RF antenna signals with each of four relative phase shifts inserted between them in succession by means of switches This requires only a single receiver, and uses one antenna as a reference antenna, combining its output with that of other antennas in turn with multiple switched phase shifts. However, the method is for estimating the bearing of a single emitter only. A patents search has indicated that the following patent documents EP455102, DE4014407, DE3636630, DE19529271 and DE2723746 are related to direction finding.
US patent application no. US 2002/0190902 A1 describes sampling RF signals from M antennas at a sampling rate equal to or greater than the signal bandwidth multiplied by 2M. This is followed by Fourier transformation of resulting signal samples to provide spectra. Direction finding is then based on line configuration in the spectra and associated phase and amplitude data. It employs a multiplicity of directional antennas to provide beamforming.
U.S. Pat. No. 4,486,757 to Ghose et al. discloses a direction finder having two separated antennas for reception of respective signals from a remote source, phase shifting one signal by 90 degrees relative to the other and nulling one signal using an error correction loop. The operation of the loop is monitored and used to calculate a bearing for the remote source. It does not appear to use receivers.
Direction finders of various kinds are also disclosed in U.S. Pat. No. 4,489,327 to Eastwell, British patent no. 1,576,616 and US patent application no. 2002/0008656 to Landt.